1. Field of the Invention
The present invention relates to a reflection-type mask, e.g., one used in extreme ultraviolet (EUV) lithography and to a method of making the reflection-type mask.
2. Background Art
In recent years, EUV lithography using extreme ultraviolet having a wavelength in the vicinity of 13.5 nm as exposure light has been proposed to realize transfer of finer patterns.
In EUV lithography, a reflection-type mask is used for reasons including small differences in absorptivity between materials to EUV light. A reflection-type mask used in EUV lithography is made on the basis of a substrate called a mask blank.
The mask blank has a mutlilayer reflective film, a buffer layer and a photoabsorber layer successively formed on a Low Thermal Expansion (LTE) grass substrate. A desired circuit pattern is formed by etching the photoabsorber layer and the buffer layer. EUV light is absorbed in a portion formed of a photoabsorber and is reflected by the multilayer reflective film in a portion formed as a result of removal of the photoabsorber. In this way, EUV light is reflected as an image of the desired circuit pattern.
A phase shifting method may be used with a reflection-type mask as well as with a transmission-type mask to further improve the resolution. For example, as such a reflection-type mask, one disclosed in Japanese Patent Laid-Open No. 2004-207593 exists. According to this publication, a mask which can have both the desired reflectance and the desired phase difference and which is capable of attenuated phase shift exposure is obtained by optimizing each of a ruthenium (Ru) layer and a tantalum (Ta) layer constituting a pattern 1 (two-layer film) formed on a multilayer film 2.
However, there is a problem that if the film thickness of the pattern formed on the mask is increased, the influence of the shadowing effect is increased.
The shadowing effect is known as a problem with lithography using a reflection-type mask. The shadowing effect is a phenomenon in which a reflected pattern image is deformed due to the height of patterned photoabsorber layer. That is, the shadow of the photoabsorber layer is cast because exposure light is incident at a certain angle (ordinarily about 6°) from a plane perpendicular to the mask blank. In the shadowed region, the reflectance with respect to the exposure light is reduced, resulting in a variation in size of the reflected pattern image.
It is possible to reduce the influence of the shadowing effect by reducing the film thickness of the photoabsorber layer. However, if the photoabsorber layer is reduced, the absorbed exposure light intensity is reduced. Therefore, the exposure light intensity transmitted through the photoabsorber layer and radiated out of the mask after being reflected by the multilayer reflective film is increased. A problem then newly arises which relates to the shading performance of a region with which shading is to be performed, e.g., a shading frame. The shading frame is a frame provided so as to surround a circuit pattern region of the mask for the purpose of preventing exposure light from leaking to a region adjacent to a pattern transfer target block on a wafer. An increased reflection light intensity from the shading frame influences the adjacent region and causes a reduction in yield. It is, therefore, desirable that the reflectance of the shading frame be sufficiently low.
There is a problem that if the film thickness of the photoabsorber layer is reduced in order to reduce the above-described shadowing effect, the intensity of light radiated from the photoabsorber layer to the outside of the mask is increased and a failure to ensure the desired shading performance of the shading frame results.